Animals and humans can become infected with more than one pathogen and the resulting clinical diseases and lesions are far greater than that capable from each pathogen alone. These are called polymicrobial diseases and originate from multiple viral infections, multiple bacterial infections, viral and bacterial infections, multiple mycotic infections, and opportunistic infections secondary to microbe-induced immunosuppression. Polymicrobial Diseases is the topic of an American Society for Microbiology sponsored conference October 2003, at the Hotel and Conference Center in Lake Tahoe, Nevada, USA. The purpose of the conference is a) to bring together a diverse group of investigators like physicians, dentists, veterinarians, molecular biologists, and researchers with backgrounds in virology, bacteriology, parasitology, and mycology, b) to discuss the complex etiology and pathogenesis of polymicrobial diseases, and c) to foster interdisciplinary research, possibly leading to productive collaborations. The format is typical of ASM sponsored conferences. Throughout the sessions and discussion periods, we have the following aims for the conference. Aim 1. To increase our understanding of the etiologic agents of polymicrobial disease. In this conference, we will identify and discuss the complex interactions among etiologic agents and the methods to study polymicrobial interactions in vitro and in vivo, which may lead to more effective diagnostic strategies. Aim 2. To increase our understanding of the determinants of polymicrobial pathogenicity and the underlying mechanisms of polymicrobial disease pathogenesis. In this conference we will identify and discuss the determinants that increase the virulence of two or more microorganisms or strains of microorganisms and delineate the common underlying mechanisms of polymicrobial disease pathogenesis. This includes identifying both host and microbial factors that increase the susceptibility of the host to multiple infections. Aim 3. To increase our understanding of immune or probiotic intervention of polymicrobial infections. In this conference, we will identify and discuss methods to prevent polymicrobial infections, which may lead to more effective prevention and treatment strategies.